PR 5.1 — Driver.Galaxy.ParityTests project shell + ParityHarness

Side-by-side fixture that boots both backends against the same dev Galaxy:

- Legacy GalaxyProxyDriver against an out-of-process Galaxy.Host EXE
  (skipped when ZB SQL on localhost:1433 isn't reachable or when the EXE
  hasn't been built).
- New in-process GalaxyDriver against an mxaccessgw gateway at
  http://localhost:5120 by default (skipped when the gateway isn't
  reachable). Endpoint, API key, and client name are env-var overridable
  for the central parity host.

Per-backend availability is independent — each scenario decides whether
to RequireBoth, GetDriver(specific), or use RunOnAvailableAsync to drive
both with the same closure and diff snapshots. PR 5.2–5.8 land scenarios
on top of this shell.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

ZB.MOM.WW.OtOpcUa.slnx

@@ -51,6 +51,7 @@
         <Project Path="tests/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.TestSupport/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.TestSupport.csproj"/>
         <Project Path="tests/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Proxy.Tests/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Proxy.Tests.csproj"/>
         <Project Path="tests/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.E2E/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.E2E.csproj"/>
+        <Project Path="tests/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.ParityTests/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.ParityTests.csproj"/>
         <Project Path="tests/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Tests/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Tests.csproj"/>
         <Project Path="tests/ZB.MOM.WW.OtOpcUa.Driver.Historian.Wonderware.Tests/ZB.MOM.WW.OtOpcUa.Driver.Historian.Wonderware.Tests.csproj"/>
         <Project Path="tests/ZB.MOM.WW.OtOpcUa.Driver.Historian.Wonderware.Client.Tests/ZB.MOM.WW.OtOpcUa.Driver.Historian.Wonderware.Client.Tests.csproj"/>